A deficiency of lactase -- an enzyme produced in the small intestine -- is responsible for lactose intolerance. Lactase is an enzyme that breaks down lactose into glucose and galactose, two simple sugars that can be absorbed and used as energy. A lactose-intolerant individual cannot properly digest lactose, and thus may experience symptoms such as diarrhea, gas, bloating and cramps after consuming foods or beverages containing the sugar. One in ten Americans has this condition -- the most common intolerance. Genetics and ethnicity affect the amount of lactase an individual produces, thereby determining the degree of lactose intolerance.
Infants' digestive systems are designed to survive on breast milk alone. To digest their mother’s milk, babies produce lactase in large amounts. As children replace milk with other foods, lactase production normally decreases. Some continue producing enough lactase to digest dairy properly; others do not, and this leads leading to primary lactose intolerance. Lactase deficiency is the most common type of lactose intolerance and usually develops before the age of 20. Infants are rarely lactose intolerant, though it is possible. Infant lactose intolerance is known as congenital lactose intolerance.
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